In the field of electrophotography, with the progress of electrophotographic systems, it has been demanded to develop toners for electrophotography adaptable for high image quality and high copying or printing speed. To meet a high image quality of the toners, there are known processes for producing toners having a narrow particle size distribution and a small particle size by an aggregating and coalescing method (emulsification-aggregation method or aggregation-coalescence method) in which fine resin particles, etc., are aggregated and coalesced together in an aqueous medium. Among them, in order to improve thermal properties such as a low-temperature fusing property and a heat-resistant storage stability, there have been proposed toners having a core-shell structure.
For example, JP 2014-13384A discloses a toner for electrophotography including core-shell particles each constituted of a core that contains a non-crystalline composite resin (A) containing a segment (A1) formed of a polyester resin and a segment (A2) formed of an addition polymer containing styrene as a constitutional unit, and a shell that contains a non-crystalline resin (B) obtained by polycondensing an alcohol component containing an aliphatic diol having 2 to 6 carbon atoms and a carboxylic acid component. In JP 2014-13384A, it is described that the toner for electrophotography is excellent in low-temperature fusing property and anti-hot offset property.
JP 2011-247932A discloses a core-shell type toner for development of electrostatic images including a core layer and a shell layer formed of a resin which covers the core layer, in which the core layer includes a core aggregate obtained by aggregating core particles containing a graft polyester resin produced by graft-polymerizing a polyester resin with a polymerizable vinyl monomer, a wax and a wax dispersing assistant. In JP 2011-247932A, it is described that the core-shell type toner for development of electrostatic images has a small particle size, and is excellent in low-temperature fusing property, separating property upon fusion, and storage stability.
Further, JP 2007-114398A discloses a toner for electrophotography having a structure including core particles each containing at least a resin binder, a colorant and a releasing agent, and a shell layer that covers the respective core particles, in which 75% by mass or more of the resin binder contained in the core particles is constituted of a polyester resin A and 75% by mass or more of the shell layer is constituted of a polyester resin B, a ratio (mol %) of isophthalic acid to a total amount of carboxylic acids constituting the polyester resin A and a ratio (mol %) of isophthalic acid to a total amount of carboxylic acids constituting the polyester resin B satisfy a specific relational formula, and solubility parameters (SP values) of the polyester resin A and the polyester resin B satisfy a specific relational formula. In JP 2007-114398A, it is described that the toner for electrophotography is not only free from problems such as exposure of internal additives of the toner to an outer surface thereof and separation of the shell layer, but also capable of satisfying a good low-temperature fusing property, a high gloss of images and a prolonged service life of a developer.
Furthermore, JP 2012-118236A discloses a process for producing a toner for electrophotography which has a sharp particle size distribution, satisfies both of a good low-temperature fusing property and a good storage stability, and is improved in toner cloud, which process includes a step (1) of melting and mixing a crystalline polyester (a) and a non-crystalline polyester (b) produced by polycondensing an alcohol component containing 80 mol % or more of a propyleneoxide adduct of bisphenol A and a carboxylic acid component, and emulsifying the resulting mixture in an aqueous medium to obtain resin particles (A); a step (2) of aggregating the resin particles (A) to obtain aggregated particles; a step (3) of adding resin particles (C) containing a non-crystalline polyester (c) obtained by polycondensing an alcohol component containing 80 mol % or more of an ethyleneoidde adduct of bisphenol A and a carboxylic acid component to a dispersion of the aggregated particles to obtain core-shell particles; and a step (4) of maintaining a system containing the core-shell particles at a temperature that is not lower than a temperature lower by 5° C. than a glass transition point of the non-crystalline polyester (c) to obtain coalesced core-shell particles.